For mode 2, 3 and 4 GPX files created by frames, add speed data

[himynamesdave]

<?xml version="1.0" encoding="UTF-8"?>
<gpx xmlns="http://www.topografix.com/GPX/1/1" xmlns:gopro2gsv="https://github.com/trek-view/gopro2gsv" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd" version="1.1" creator="gpx.py -- https://github.com/tkrajina/gpxpy">
  <trk>
    <trkseg>
      <trkpt lat="LAT" lon="LON">
        <ele>ELE</ele>
        <time>TIME</time>
        <extensions>
          <gopro2gsv:InputPhoto>PATH TO PHOTO</gopro2gsv:InputPhoto>
          <gopro2gsv:SpeedMStoNext>SPEED TO NEXT PHOTO</gopro2gsv:SpeedMStoNext>
          <gopro2gsv:DistanceMtoNext>SPEED TO NEXT PHOTO</gopro2gsv:DistanceMtoNext>
          <gopro2gsv:TimeSecstoNext>SPEED TO NEXT PHOTO</gopro2gsv:TimeSecstoNext>
        </extensions>
      </trkpt>